Electronic fuel control systems are increasingly being used in internal combustion engines to precisely meter the amount of fuel required for varying engine requirements. Such systems vary the amount of fuel delivered for combustion in response to multiple system inputs including throttle angle and the concentration of oxygen in the exhaust gas produced by combustion of air and fuel. Typical electronic fuel control systems operate in a closed-loop mode in response to sensed exhaust oxygen level in order to maintain the ratio of air and fuel at or near stoichiometry. Improved forms of fuel control systems include an adaptive mechanism which learns and remembers the probable amount of fuel that needs to be injected under previously experienced engine operating conditions which can later be identified by the values of such sensed variables such as engine speed, engine load, engine coolant temperature and fuel type.
The speed at which the electrically actuated fuel injectors can respond to a given control signal, and hence the amount of fuel injected in response to that signal, has been found to be influenced by the battery voltage level available to the injector actuators. Accordingly, conventional liquid fuel injection systems of the type used with engines fueled by gasoline, methanol, and ethanol have employed processing means for altering the duration of the fuel injection command signals to compensate for the effects of battery voltage on fuel delivery.
The fuel control systems designed for use in liquid-fueled vehicles have proven to have significant shortcomings when used to supply compressed gases, such as natural gas, rather than gasoline and other liquid fuels.
First, while liquid-fuel pressure regulators are typically capable of delivering fuel at a substantially constant pressure, typically about 40 p.s.i.g., over a wide range of operating conditions, regulators used with compressed natural gas supplied at higher pressure, typically around 100 p.s.i.g., cannot maintain constant pressure over the wide range of operating temperatures and flow rates to which the engine is subjected.
Secondly, variations in ambient temperature at the injectors change the resistance exhibited by the actuating solenoids, and that resistance change in turn alters the time required for the injectors to respond to actuating signals, resulting in substantial variation in the amount of fuel delivered by the injectors.